For machine parts such as engine parts and underbody parts of automobiles or motorcycles, steel materials are used, to which the induction hardening is performed, namely the metal (conductive material) to which high-frequency induction heating is performed. As to the high-frequency hardening to the steel material, case depth after hardening by the surface hardening (hereinafter called “hardening depth”) and hardness are standardized in effective case depth after hardening and total case depth after hardening. Thus, in order to guarantee quality of the steel material, the hardening depth and the hardness are required to be measured and evaluated.
The hardening depth and the hardness of the steel material are conventionally evaluated by measurement with Vickers hardness tester or the like, which measures the strength of a cross section of a steel material as a sample.
Due to the destructive inspection, the steel material as the sample is wasted and therefore the material cost rises. Moreover, it takes long time to inspect and the total inspection in the in-line inspection would be impossible, and the defects accidentally occurred are not detected and the product with defects may be transferred to the next process.
There are some techniques using eddy current inspection (measurement) as a non-destructive inspection to measure the hardening depth and the hardness of the steel material (e.g., Patent Literatures 1 to 6 and Non-Patent Literature 1).
The eddy current inspection is to generate an alternating magnetic field by putting an exciting coil with alternating currents close to the steel material, generating eddy currents in the steel material caused by the alternating magnetic field, and to detect an induction magnetic field induced by the eddy currents with a detection coil. The eddy current inspection makes it possible to prevent waste and to measure the hardening depth and hardness of the steel material in total inspection in a short time.
Such eddy current inspection is used not only for measuring the hardening depth and hardness of the steel material (hereinafter called “hardening depth inspection”) but also for detecting flaws such as cracks on the surface of the object or detecting foreign matters contained in the object.
In the steel material, there occurs the difference in electric conductivity between the base layer and the hardened layer (martensitic layer). Therefore, when measuring the steel material with the eddy current sensor, the voltage, or the amplitude, detected by the detection coil differs in accordance with the change of the hardening depth. Furthermore, the voltage monotonically decreases in accordance with increase of the hardening depth. The hardening depth inspection uses such phenomenon to calculate the hardening depth of the steel material.
For instance, JP 2009-236679 A (PTL1) uses an annular coil to detect the hardening depth of axis of a shaft member. The annular coil can generate the stronger magnetic field than a probe coil that measures the hardening depth with a coil perpendicular to the steel material, and the distance between the annular coil and the steel material may not be strictly controlled, so that the annular coil is useful for the hardening depth inspection.
The steel material must be inserted into the annular coil, and the object is limited to the shaft member having the constant outer diameter. In other words, the annular coil cannot be applicable to the steel material having the complicated inside shape in which the hardening is performed three-dimensionally such as an outer race for a constant velocity joint (“CVJ outer race”).
JP 2007-40865 A (PTL2) discloses the technique of measuring the hardening depth with the probe coil.
JP 2006-10440 A (PTL3) discloses the magnetic sensor having multiple coils in which the intervals between the coils are not constant.
JP 2008-185436 A (PTL4) discloses the device for identifying a characteristic of a metal member including four detection coils and an exciting coil.
JP 2009-2681 A (PTL5) discloses the device for measuring the magnetic in which the detection coil is movable.
JP 2003-185758 A (PTL6) discloses the sensor including three detection coils.
NPTL 1 discloses the technique of introduction for mass production of in-line inspection for hardening depth using the eddy current inspection.
As to the hardening depth inspection, the rate of detection signal with respect to that of noise is smaller than the other inspections, so that the hardening depth inspection needs higher detection accuracy. In the case where the object is the steel material such as the CVJ outer race having the complex inside shape in which the hardening is performed three-dimensionally, stability and repeatability are required in positioning the probes, and therefore it is difficult to utilize the eddy current inspection.
[Patent Literature 1] JP 2009-236679 A
[Patent Literature 2] JP 2007-40865 A
[Patent Literature 3] JP 2006-10440 A
[Patent Literature 4] JP 2008-185436 A
[Patent Literature 5] JP 2009-2681 A
[Patent Literature 6] JP 2003-185758 A
[Non-Patent Literature 1] Takanari Yamamoto and Tetsuya Yamaguchi “Development of Hardening Depth Evaluation Technique using Eddy Current-Establishment and Introduction of In-line Hardening Depth Inspection System-”, SAE World Congress, SAE Paper 2009-01-0867, 2009